This invention relates to improved field-effect transistors, especially those used in applications demanding high-frequency and/or high-power output, typically microwave field-effect transistors.
Field-effect transistors (FET's), including microwave FET's, have been in existence for some time. See, e.g., J. V. DiLorenzo and D. D. Khandelwal, GaAs FET Principles and Technology, Artech House, 1982, whose disclosure is incorporated by reference herein. One well-known microwave device uses single crystal GaAs (gallium arsenide) or InGaAs (indium gallium arsenide) as a channel (or conduction material). In these materials, the electron carriers suffer from the Gunn effect as shown in FIG. 1. The carriers "slow down" when a large field is applied to the device. This effect is due to scattering of electrons, e.g., by phonons, into higher energy valleys which have poorer transport properties associated therewith. This "slowing down" of the carriers is detrimental to the frequency response of the device because the transit time of electrons is given by: EQU Frequency Response=V/A (1)
The power output of a device (per unit area) is proportional to voltage.times.current or: EQU Power Output=(A)(E)(v)(n) (2)
where A=channel length; E=electric field; v=average velocity; and n=carrier density. For a given channel length, A, Equations (1) and (2) demonstrate the desirability of having both the highest velocity, v, and the highest field, E, for a good high frequency, high power amplifier. FIG. 1 shows that this condition cannot be met simultaneously in conventional GaAs and InGaAs FET's because of the Gunn effect.
Other prior art microwave FET devices have extremely short channel lengths. In these devices, a velocity overshoot or ballistic transport can occur and the Gunn effect can be overcome (DiLorenzo and Khandelwal, supra, p. 735). However, due to the short length, A, these are (from Equation 2) low power devices.
The High Electron Mobility Transistor (DiLorenzo and Khandelwal, supra, p. 741) is another device wherein a heterojunction is used to spearate the doped and undoped regions of the channel to provide very high electron mobilities (corresponding to applied fields below 5 kV/cm in FIG. 1). The electrons flow in a very thin layer (a two-dimensional electron gas) at the heterojunction interface. These are not power devices because of the thin conducting layer and because they suffer the Gunn effect at high fields.
Another restriction in prior art FETs is the choice of substrate materials on which a FET is fabricated. Lattice mismatches &gt;0.1% result in very poor material quality. Most prior art microwave FETs are made from GaAs or AlGaAs on GaAs substrates, or InGaAs on InP substrates because these materials are lattice matched. Very few such matches exist for semiconductor combinations otherwise satisfying the requirements of various prior art devices.
U.S. Pat. No. 4,163,237 discloses an FET having a lattice matched superlattice as its channel material. The device is based on the technique of modulation doping. It possesses improved low field transport at low temperatures. No mention is made of improved high-frequency response or of improved carrier velocity at high electric fields.